For some applications, such as for optical fiver waveguide manufacture, very high purity glasses are required in which the impurity levels of less than 0.1 p.p.m. are desired. The normal manufacture of such glasses requires the use of high purity source materials in powder form.
For materials that are water soluble, such as the carbonates of potassium and sodium, high purity can be obtained by fractional crystallization. A modified version of the same method may also be used to prepare calcium carbonate, which is not soluble in water. In this case a soluble calcium salt is first purified by fractional crystallization, and then the carbonate is obtained by precipitation from solution by the addition to the solution of a soluble carbonate.
Other source materials, such as silica, alumina, boric oxide, and phosphorus pentroxide are not readily obtainable by fractional crystallization, but can be obtained by the oxidation of pure volatile compounds that have been purified by fractional distillation. Many of these volatile compounds have been prepared in exceptionally pure forms to meet the existing demands of semiconductor technology. However, although it is possible to produce these oxides in pure form by a chemical vapor reaction, the product is usually particularly liable to contamination during subsequent handling. This is because either the product is produced in the form of a boule which is liable to contamination when being crushed to form a powder of suitable grain size for glass making, or the product is produced in the form of a very fine, low bulk density, absorbent, and highly reactive powder that is liable to become contaminated with any inpurity to which it may become accidentally exposed. Grain size is particularly important where a high degree of homogeneity is essential.
Nevertheless in respect of the more commonly used glass making source materials, such as boric oxide and silica, the need for very pure materials has been sufficiently long standing and sufficiently extensive for technologies to have been developed to produce these materials with a high degree of purity and a grain size suitable for glass manufacture. It should be appreciated that a particular powder may be adequately pure for use in making one glass composition for one application, while it may be unacceptable for another glass composition having the same or differing overall purity requirements. One significant factor will be the proportion of that material that is being called for in the composition being made.